This invention relates to a measuring system employing a magnetic scale, and more particularly to an improvement of a multi-gap head for use with the magnetic scale for sensing the magnetization patterns thereof and changing them into electric signals.
As is known, the measuring system for measuring a length, angle and so forth, by using a magnetic scale, has been employed in various fields because of its high degree of accuracy, and it has widely been applied in the field of automatic control systems recently. Such a measuring system includes a magnetic scale having magnetization patterns formed with a fixed pitch thereon and a magnetic head disposed in an opposed relation to the magnetic scale. Either the scale or the head is fixed, and the other is attached to an object, the displacement of which is to be measured. The length of travel of the object is measured by electrically processing the signals which are produced by the magnetic head when the object moves.
The measuring system, however, necessitates meeting the following requirements. Measurement should be possible even if the relative velocity between the scale and the head is not constant and the object remains static. Further, the pressure between the scale and the head should not be great, so as to prevent wear thereof.
A magnetic flux sensing type head is used in order to meet the above requirements. The exciting core of the magnetic flux sensing type head has an exciting winding and a signal winding. High frequency exciting current is supplied to the exciting winding. The signals are obtained from the signal winding when the exciting current is modulated by the magnetization patterns of the scale. In this case, a multi-gap type core has been recently used as the exciting core in order to obtain high accuracy and high output. The exciting core of the multi-gap type core comprises a plurality of thin plates made of a high permeability material such as permalloy, and thin plates made of a non-magnetic material such as beryllium copper. The two kinds of plates are stacked alternately to make a core member in which the plates made of the high permeability material may be disposed with fixed gaps therebetween. The thickness of the core corresponds with the pitch .lambda. of the magnetization patterns formed on the magnetic scale. The exciting core has two cores having n core members, and the cores are so disposed that the gap therebetween may become n.lambda..+-.(1/4 ).lambda..
In the above-mentioned multi-gap head, the gap between each plate made of the high permeability material needs to be restrictively determined because the gap is microscopic and affects the intrinsic wave length of the head.
Therefore, in the prior art, the gaps between the plates made of a high permeability material is kept at a fixed value by so restrictively determining the thickness of both plates made of a high permeability material and those made of a non-magnetic material within a fixed range. For example, in the case that the gap between the plates made of high permeability material is determined as 100 .mu.ms, the thickness of the plates both made of high permeability material and of non-magnetic material is accurately determined as 50 .mu.ms.
The manufacture of the multi-gap head, therefore, requires much trouble. For example, the thickness of the plates should be controlled with care and the work for piling the plates should also proceed with care. Particularly, the thin plates are gained by stamping out the rolled plates made of high permeability and non-magnetic materials. For example, when plates with a thickness of 50 .mu.ms are made, the plates are produced with a thickness error within .+-.5 .mu.ms, and a thickness error within .+-.2 .mu.ms occurs even if the materials are well rolled. As a result, for example, in the case that the core member is composed of m plates with the thickness error within .+-.5 .mu.ms, the dimensional error of the core within .+-.5 .mu.ms results therefrom.